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The seasons begin at the time of the solstice or equinox.

(This is a newer, updated version of this page. I am hoping
that this newer version will make more sense to folks, and that
the amount of email I get from this page will ease up a bit! ;-) )
I think I will break from my usual format of
Bad Astronomy/Good Astronomy by saying that the
way we define seasons currently is not strictly bad, but I feel
(in my opinion!) that it could be better. The definition of when the
seasons begin is at the moment of solstice or equinox;
that is, winter (in the north) starts on December 22nd
and summer starts on June 22nd.
I feel instead that the midpoint of the seasons are really at these
times. The seasons themselves start a month and a half before then.

How it works:
During a local news broadcast, they said that winter begins
in December. I prefer to think that the Winter Solstice is in December,
and winter itself started a month and half earlier.
We will need to have a few definitions here, so hang on!

First off, I am in the Northern hemisphere, so I will make all
my descriptions using that reference. I apologize to my Southern
readers; just reverse the references to the seasons (i.e., replace
"winter" with "summer") and it should straighten things out.

The Earth orbits the Sun in one year (by definition!).
The seasons are caused by the tilt
of the Earth to the plane of its orbit. The Earth is tilted about
23 degrees, so that sometimes the North pole is tipped toward the Sun,
and other times the South pole is pointed more toward the Sun. From here
on the Earth, this means the Sun moves north and south about 46 degrees
over the course of the year. In the summer, the Sun is very high in the
sky, but in the winter it never gets as high; the difference is that
very same 46 degrees (about 1/4 of the way around the visible part of the
sky!).

Now imagine breaking up the Sun's 46 degree up-and-down yearly swing into 1 day
intervals. Start at the top; then 3 months later it is midway between
extremes and heading south, then 3 months after that it is at the lowest point
(6 months after the peak). After that, it starts heading back up;
three months later it is at the
midway point on its way up. A full year after we start, it is back at the
top of its journey. We have names for these four times; in order, they are
the Summer Solstice, the Autumnal (or Fall) Equinox, the Winter Solstice,
and the Spring (or Vernal) Equinox).

Now, bear with me: if summer started at the moment the Sun hit its peak,
then every day of summer would mean the Sun gets a little lower in the sky.
This to me doesn't make sense. You want the Sun to hit its peak at summer's
midpoint, meaning summer would start 1.5 months earlier. That way the Sun would
be climbing steadily higher until the peak of summer, then start to fall. This
would in turn mean that the Sun hits its nadir (lowest point) in the middle
of winter, and not at winter's beginning! See?

[Note (added July 31, 1998): As it happens, a lot of countries do
actually think of the seasons this way; Japan for example. A lot of European
countries do too; they have a Midsummers Day (made famous by William Shakespeare)
on June 21 or so, and the days I claim should be the actual season starters
are called "cross-quarter" days. I'll note that I didn't know this when I
started the page. I'll also note that the email I get complaining about
my opinion on this is overwhelmingly from people in the US, who also use
weather as an argument. I specifically say here that I am avoiding
using weather in my reasoning. Also, the email that supports my idea usually
comes from outside the US boundaries.]

The length of the day depends on this effect as well. When the Sun is at the peak
of its path, the day is at its longest (think of it this way: it has more
distance to travel to get across the sky!). When the Sun is at its lowest, the day is
shortest. It makes far more sense to me to have the longest day of the year at mid-summer,
and not the first day, much as the shortest day should be mid-winter.

There is one major drawback with this definition of the seasons.
Most people like to base definitions on something, well-- definite.
That's probably the reason we define the seasons at such specific,
easily measured dates. My definition would change that so that the
mid-season is defined at these times, and thus the beginnings
of the seasons would not have such a definite start and stop.

The point I am trying to make here is that the seasons are
defined by an astronomical event-- the crossing of the Sun over
the Celestial Equator (see below). This event does not really tie into the
weather at all: by December where I live, things have been cold for over
a month and will stay that way for at least two more! So I
am trying to avoid the whole issue of weather and simply say that
the definition of seasons -- while fine in and of itself--
could be tied into our daily lives better by using the
length of the day as a guide.

This makes
some people uncomfortable, as my email has been telling me! Still,
I am not trying to change the world here-- just pointing out that maybe
our current definition needs some background to explain why we chose it.
Now mind you, I don't have a lot of scientific backing on this claim, just my own
personal feelings on symmetry, beauty and common sense.

Bad Addendum (December 11, 1996): I have received quite a bit of mail
about this. The messages tend to fall into two groups:
one uses weather as a guide to
the seasons, and the other talks about definitions.
Let me make this clearer now: I am talking
about the somewhat arbitrary definition of seasons, not the climatic
effects. Temperature
changes with seasons are highly latitude-dependent: someone in Ecuador, for
example, sees little temperature change year round, while in Washington DC
(trust me here!) the change is dramatic (well over 30 degrees Centigrade).
So I am avoiding that issue completely. The actual definition of
seasons is purely astronomical: the Equinoxes are defined as the two points
on the sky where the path of the Earth around the Sun (the ecliptic)
intersects the projected equator of the Earth on the sky (the Celestial
Equator or CE). These two points are generically called nodes. As the
Sun travels around the ecliptic it moves generally in an East/West direction,
with some small North/South motion due to the inclination of the ecliptic to
the CE. When the Sun hits the Vernal Equinox, it is moving
North (on its way up in the sky as I discussed above). Six months later,
it is moving South (on its way down) when it reaches the Autumnal Equinox.
At the Summer Solstice the Sun has reached its highest point (the farthest
North of the CE), and at the Winter Solstice its lowest point (the farthest
South of the CE). That is how the seasons are actually defined!
(phew!) I know this is confusing and awful without a diagram, but
I don't have good software yet to make the diagrams I need for this page. I will
include them as soon as I can draw them!